Imaging system with intermediate transfer members

ABSTRACT

Imaging apparatus for printing an image on a substrate (42) from a latent image formed on a latent image bearing surface (16) including developing apparatus (22) for developing the latent image/with toner to form a developed toner image of a given size, a first intermediate transfer member (40) having a surface area large enough to accomodate the developed toner image, first transfer means for transferring the developed toner image from the latent image bearing surface (16) to the first intermediate transfer member (40), a second intermediate transfer member (47) having a surface area smaller than the surface area of the first intermediate transfer member (40), and second transfer means for transferring of said developed image from said first intermediate transfer member (40) to said second intermediate transfer member (47) and from said second intermediate transfer member (47) to said substrate (42). Preferably the second intermediate transfer means (47) is a cylinder having a diameter of less than 30 or 40 mm.

FIELD OF THE INVENTION

The present invention relates to image transfer techniques and apparatusfor use in electrophotography.

BACKGROUND OF THE INVENTION

Various prior publications deal with the transfer of single and multiplepowder and liquid toner images from a photoreceptor on which they areformed to an intermediate transfer member for subsequent transfer to afinal substrate.

U.S. Pat. No. 3,838,919 to Takahashi describes a powder toner system inwhich color toner images are sequentially formed on an image formingmember, individually transferred to an intermediate transfer member andtransferred at one time to a recording member.

U.S. Pat. No. 4,144,808 to Isawa et al. describes a method of printingon a metal plate utilizing powder toner and an intermediate transfermember where the plate is heated before transfer.

U.S. Pat. No. 4,518,976 to Tarumi et al. describes a monochrome powdertoner system in which a powder image is developed on a photoreceptor,and transferred electrostatically to an intermediate transfer member.Downstream this transfer, the intermediate transfer member and the imagethereon are heated before transfer to a preheated substrate.

U.S. Pat. No. 4,515,460 to Knechtel, describes a powder toner apparatuswherein separate toner images are sequentially developed on aphotoreceptor and electrostatically transferred to an intermediatetransfer member. After all of the individual images have beentransferred to the intermediate transfer member, they are transferredelectrostatically to the final substrate. No heating of the images orsubstrate is disclosed.

U.S. Pat. No. 4,585,319 to Okamoto et al. describes a powder developertype, single color system, utilizing a temperature controlledphotoreceptor, a heated intermediate transfer member and a heatedtransfer fixing roller which is heated to a temperature slightly higherthan that of the intermediate transfer member.

U.S. Pat. No. 4,690,539 to Radulski et al. describes a liquid tonermulti-color system in which a color image is developed on aphotoreceptor and transferred to a belt type intermediate transfermember. The liquid carrier is removed from the toner image on the belt.There is no mention of heating the intermediate transfer member or ofthe problem of back transfer.

U.S. Pat. No. 4,708,460 to Langdon describes a single color liquid tonersystem in which a developed image is transferred from a photoreceptor toan intermediate transfer member, heated on the transfer member and thentransferred to a final substrate.

U.S. Pat. No. 3,847,478 to Young describes a duplex printing system,wherein a developed image is transferred from a photoconductor to anintermediate transfer member, a second image is developed on thephotoconductor and both images are transferred electrostatically toopposite sides of a piece of paper passed between the intermediatetransfer member and the photoreceptor.

SUMMARY OF THE INVENTION

The present invention seeks to provide improved apparatus for thetransfer of an image from an image bearing surface to an intermediatetransfer member and subsequent transfer to a final substrate.

There is thus provided in accordance with a preferred embodiment of theinvention imaging apparatus for printing an image on a substrate from alatent image formed on a latent image bearing surface includingdeveloping apparatus for developing the latent image with toner,preferably with liquid toner having carrier liquid and toner particles,to form a developed toner image of a given size, a first intermediatetransfer member, preferably having a cylindrical shape and having asurface area large enough to accommodate the developed toner image,first transfer apparatus for transferring the developed toner image fromthe latent image bearing surface to the first intermediate transfermember, a second intermediate transfer member, preferably a cylindricalshape having a surface area smaller than the surface area of the firstintermediate transfer member and second transfer apparatus fortransferring of the developed image from the first intermediate transfermember to the second intermediate transfer member and from the secondintermediate transfer member to the substrate.

Preferably the second intermediate transfer member is not large enoughto accomodate the developed image.

In a preferred embodiment of the invention the imaging apparatusincludes heating apparatus for heating the first intermediate transfermember to a first temperature and for heating the second intermediatetransfer member to a second temperature higher than the firsttemperature.

Preferably the second transfer apparatus includes apparatus for heatingthe substrate, preferably including a heating backing roller operativeto apply heat and pressure to the image during image transfer to thesubstrate.

In a preferred embodiment of the invention the imaging apparatus alsoincludes first voltage apparatus for maintaining the first intermediatetransfer member at a first voltage. Preferably at least a portion of thelatent image bearing surface is at a second voltage and the firstvoltage is different from the second voltage. Preferably the imagingapparatus also includes second voltage means for maintaining the secondintermediate transfer member at a third voltage.

Preferably the second intermediate transfer member has a diameter ofless than about 40 mm, more preferably a diameter of less than about 30mm.

In a preferred embodiment of the invention transfer of the developedimage from the second intermediate transfer member to the substratecommences before transfer of the developed image from the firstintermediate transfer member to the second transfer member is complete.

In a preferred embodiment of the invention the imaging apparatusincludes means for producing a plurality of developed images on theimage bearing surface and for transferring the plurality of developedimages to the first transfer member in mutual alignment thereon.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will be understood and appreciated more fully fromthe following detailed description, taken in conjunction with thedrawings in which:

FIG. 1 is a simplified sectional illustration of electrophotographicapparatus constructed and operative in accordance with a preferredembodiment of the present invention;

FIG. 2 is a simplified sectional illustration of electrophotographicapparatus constructed and operative in accordance with another preferredembodiment of the present invention;

FIG. 3A is a simplified sectional illustration of electrophotographicapparatus constructed and operative in accordance with yet anotherpreferred embodiment of the present invention;

FIG. 3B is a simplified sectional illustration of electrophographicapparatus constructed and operative in accordance with yet anotherpreferred embodiment of the present invention;

FIG. 4 is a simplified sectional illustration of a electrophotographicapparatus constructed and operative in accordance with yet anotherpreferred embodiment of the present invention;

FIG. 5 is a simplified sectional illustration of electrophotographicapparatus constructed and operative in accordance with yet anotherpreferred embodiment of the present invention;

FIG. 6 is a simplified sectional illustration of electrophotographicapparatus constructed and operative in accordance with yet anotherpreferred embodiment of the present invention; and

FIG. 7 is a graphical illustration of the temperature variation along alow thermal mass intermediate transfer member in an arrangement such asthat illustrated in FIG. 6.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

Reference is now made to FIG. 1 which illustrates electrophotographicimaging apparatus constructed and operative in accordance with apreferred embodiment of the present invention. This and otherembodiments of the invention are described in the context of liquiddeveloper systems with negatively charged toner particles and positivelycharged photoreceptors. Such systems operate in a "write-white" mode,for which areas which are not to be toned are exposed to light. Theinvention may be useful for other combinations of toner charge,photoreceptor charge as well as for other writing systems, such as"write-black" systems.

The apparatus of the invention is described using a liquid developersystem. In accordance with a preferred embodiment of the invention theliquid developer of Example 1 of U.S. Pat. No. 4,794,651 can be used,but other suitable developers may be used in the practice of theinvention. Especially useful are liquid developers comprising tonerparticles which solvate the carrier liquid of the developer at elevatedtemperatures, above room temperature.

As in conventional electrophotographic systems, the apparatus of FIG. 1comprises a drum 10 arranged for rotation about an axle 12 in adirection generally indicated by arrow 14. Drum 10 is formed with acylindrical photoreceptor surface 16.

A corona discharge device 18 is operative to generally uniformly chargephotoreceptor surface 16 with a positive charge. Continued rotation ofdrum 10 brings charged photoreceptor surface 16 into image receivingrelationship with an exposure unit including a lens 20. Lens 20, focusesa desired image, which may be laser generated, onto chargedphotoreceptor surface 16, selectively discharging the photoreceptorsurface, thus producing an electrostatic latent image thereon.

Continued rotation of drum 10 brings charged photoreceptor surface 16bearing the electrostatic latent image into operative association with adevelopment unit 22, operative to apply a liquid developer to developthe electrostatic latent image. For multicolor copying or printing, thedevelopment unit 22 can, for example, comprise a plurality ofdevelopers, one for each color, which are selectively engaged with thephotoreceptor, as described, for example, in U.S. Pat. No. 4,690,539,which is incorporated herein by reference, or a single developmentstation where the liquid toner is changed between colors, or any othersuitable development system. In general this development process takesplace at a relatively low temperature, namely approximately thetemperature of the environment of the system.

In accordance with a preferred embodiment of the invention, followingapplication of toner thereto, photoreceptor surface 16 passes atypically positively charged rotating roller 26, preferably rotating ina direction indicated by an arrow 28. Roller 26 functions as a meteringroller and reduces the thickness of liquid on photoreceptor surface 16.Typically the spatial separation of roller 26 from photoreceptor surface16 is about 50 microns.

Preferably the voltage on roller 26 is intermediate the voltages of thelatent image areas and of the background areas on the photoreceptorsurface. Typical voltages are: roller 26: +200 V, background area: +50 Vand latent image areas: up to about +1000 V.

Liquid which passes roller 26 should be relatively free of pigmentedparticles except in the region of the latent image.

Downstream of roller 26 there is preferably provided a rigidizing roller30. Rigidizing roller 30 is preferably formed of a resilient polymericmaterial, for example a slightly conductive resilient polymeric materialas described in either or both of U.S. Pat. Nos. 3,959,574 and 3,863,603the disclosures of which are incorporated herein by reference. Roller 30is preferably resiliently urged against photoconductive surface 16.

In a preferred embodiment of the invention, an electrically biasedsqueegee roller is used as roller 30. Roller 30 is negatively charged toa potential of at least several hundred and up to 2000 volts with thesame sign as the charge on the pigmented toner particles, so that itrepels similarly charged pigmented particles and causes them to moreclosely approach the image areas of the photoreceptor surface 16, thuscompressing and rigidizing the image.

Downstream of rigidizing roller 30 there is provided an intermediatetransfer member 40, which rotates in a direction opposite to that ofphotoreceptor surface 16, as shown by arrow 41, providing zero relativemotion between their respective surfaces at the point of propinquity.Intermediate transfer member 40 is operative for receiving the tonerimage from photoreceptor surface 16 and for transferring the toner imageto a receiving substrate 42, such as paper. Disposed internally ofintermediate transfer member 40 there may be provided a heater 46, toheat intermediate transfer member 40.

Various types of intermediate transfer members are known and aredescribed, for example in U.S. Pat. No. 4,684,238, PCT Publication WO90/04216 and U.S. Pat. No. 4,974,027 the disclosures of all of which areincorporated herein by reference.

Following the transfer of the toner image to intermediate transfermember 40, photoreceptor surface 16 engages a cleaning station 49. Thisstation may be any conventional cleaning station, comprising a cleaningroller 50 which may comprise a suitable resilient material such as foampolyethylene or neoprene. Cleaning roller 50 may be wetted by cleanlubricating cleaning liquid, which preferably comprises liquid developerfrom which all or nearly all of the toner particles have been removed.Cleaning roller 50 is driven so that its surface moves opposite tosurface 16 at their nip, to provide scrubbing action for removal ofresidual particles and carrier liquid from photoreceptor surface 16. Ascraper 56 completes the removal of any residual toner which may nothave been removed by cleaning station 49.

A lamp 58 completes the cycle by removing any residual charge,characteristic of the previous image, from semiconductor surface 16.

Transfer of the image to intermediate transfer member 40 is preferablyaided by providing electrification of intermediate transfer member 40 toa voltage opposite that of the charged particles, thereby causingtransfer by electrophoresis. It has been found by the inventors, that,at least for the preferred developer, raising the temperature of thedeveloped toner image to a temperature higher than the developmenttemperature and room temperature aids this first transfer, even when thetransfer is by electrophoresis.

Subsequent final transfer of the image from intermediate transfer member40 to substrate 42 is preferably aided by heat and pressure. A highertemperature than that used for first transfer is preferably utilized forthis subsequent final transfer, in accordance with the presentinvention.

In the prior art a liquid toner image was first transferred to anintermediate transfer member. The toner image was heated during theinterval between first and second transfer so as to aid in finaltransfer.

In the present invention the preferred first transfer step, i.e., thetransfer of the liquid toner image to the intermediate transfer memberincludes the heating of the image either before or during firsttransfer. The preferred final transfer step, i.e., the transfer of theliquid toner image to the final substrate, includes the further heatingof the image before and/or during second transfer. This further heatingcan be achieved by heating the image on intermediate transfer member 40,for example by heat transfer from intermediate transfer member 40 duringthe interval between first and final transfer and/or by external heatingof the image. Preferably the image is heated to a temperature at whichit solvates liquid to form a single phase, without evaporatingsubstantial amounts of liquid carrier. Alternatively or additionally thefurther heating can be achieved by conduction heating of the image fromthe final substrate during final transfer.

These preferred first and second transfer steps improve the quality ofthe image on the final substrate both for single color and formulti-color images.

For multicolor systems it is useful to sequentially transfer theseparate colors to intermediate transfer member 40 in alignment with andgenerally superimposed and in registration with each other and then totransfer them together to paper or other substrate 42. It has then beenfound that for this configuration, there is a tendency for the heatedimages previously transferred to the intermediate transfer member at alower temperature, to transfer back, in whole or in part, tophotoreceptor surface 16, when the previously transferred image returnsto the point of first transfer.

The embodiments of the invention described herein provide improved firstand final transfer and for multicolor systems can solve the backtransfer problem.

In general, some of the embodiments of the invention are characterizedin that photoreceptor 16 is at a first, relatively low temperature;intermediate transfer member 40 is at a second, somewhat highertemperature, to provide for improved first transfer; and final substrate42 is at a third, even higher temperature to provide for good transferfrom intermediate transfer member 40 to substrate 42.

Alternatively or additionally, some of the embodiments can becharacterized in that, when a toner image is transferred fromphotoreceptor surface 16 to intermediate transfer member 40, and then tofinal substrate 42, the toner image is hotter during transfer to theintermediate transfer member than it was on the photoreceptor surfaceand the image is hotter when it is transferred to the final substrate,than during the earlier transfer.

Alternatively or additionally, some of the embodiments can becharacterized in that, when multiple toner images are transferredsequentially from photoreceptor surface 16 to intermediate transfermember 40, and then to final substrate 42 as a group, the composite,multicolor toner image is hotter when it is transferred to the finalsubstrate than during any contact of earlier transferred images with thephotoreceptor.

One embodiment of the invention can be characterized in that the imageis transferred from a photoreceptor surface, at a first relatively lowtemperature to a first intermediate transfer member at a secondintermediate temperature. The image is then transferred to a secondintermediate transfer member. Final transfer takes place from the secondintermediate transfer member to the final substrate at a third, highertemperature. Preferably, the image temperature during first transfer ishigher than that of that portion of the photoreceptor surface not incontact with the intermediate transfer member.

Returning now to FIG. 1, intermediate transfer member 40 is heated to atemperature sufficient to enhance the electrophoretic transfer of tonerparticles from photoreceptor surface 16 to intermediate transfer member40. The image is heated during transfer to intermediate transfer member40, and the heating continues while the image is on intermediatetransfer member 40 until the image is at the temperature of intermediatetransfer member 40. Rotation of intermediate transfer member 40 bringsthe heated intermediate transfer member 40 into image transferrelationship with a final substrate 42, which is pressed against theintermediate transfer member by a heated backing roller 43. Heatedbacking roller 43 heats the paper and thereby heats the image in contacttherewith by conduction from the paper, to a sufficient degree to ensurethat complete or nearly complete final transfer of the image to thesubstrate, by heat and pressure, takes place.

While the invention has been described in a monochromatic version, whereit gives improved transfer from the photoreceptor to the intermediatetransfer member and from the intermediate transfer member to the finalsubstrate, the invention is particularly useful in a multicolor system,wherein images of different colors are sequentially formed onphotoreceptor surface 16, and transferred one by one in mutual alignmentto image transfer member 40 prior to a single transfer of all of theimages, which form a multicolor image, to final substrate 42.

Final substrate 42 is brought into transfer engagement with intermediatetransfer member 40 only when all of the colors have been transferred tointermediate transfer member 40, for final transfer of multicolor imageto substrate 42.

As noted above, it is appreciated that during first transfer ofsubsequent images from photoreceptor surface 16 to image transfer member40, earlier transferred images return to the region of first transfer.Any back transfer of previously transferred images to photoreceptorsurface 16 will result in undesirable artifacts in the final printedimage.

Generally if the intermediate transfer member is heated to a temperaturewhich is useful for good final transfer, then there is tendency for theimage to back transfer to the photoreceptor.

The arrangement of FIG. 1, with proper choice of temperatures forintermediate transfer member 40 at first transfer, and for finalsubstrate 42 and the image at second transfer in accordance with thepresent invention, substantially eliminates the problem of back transferto photoreceptor surface 16, by keeping the image temperature, when theimage on the intermediate transfer member returns to the photoreceptor,low enough so that it is not tacky enough to stick to the photoreceptor.

FIG. 2 shows a second embodiment of the invention in which all of theparts and operation are generally the same as those of the apparatus ofFIG. 1, except that heated backing roller 43 is replaced by an unheatedbacking roller 44, and final substrate 42 is preheated by heating lamp45. A combination of the embodiments of FIGS. 1 and 2 is also useful,whereby paper 42 is pre-heated by lamp 45, and heated roller 43 is used.

A third embodiment of the apparatus of the invention is shown in FIG.3A. In this case intermediate transfer member 40 is heated to a first,moderate, temperature which is high enough to enhance first transfer,but not so high as to cause substantial back transfer of previouslytransferred images from intermediate transfer member 40 to photoreceptorsurface 16. The images are transferred to a second intermediate transfermember 47 which is heated by an internal heater 48 to a highertemperature, sufficient to assure good final transfer to final substrate42.

In a preferred embodiment of the invention, intermediate transfer member40 is maintained at a first voltage (different from the voltage of thephotoreceptor surface 16) to enhance transfer of the image thereto fromphotoreceptor surface 16, and second intermediate transfer member 47 iselectrified to a second voltage, different from the first voltage, toenhance transfer of the image thereto from intermediate transfer member40.

Transfer to second intermediate transfer member 47 can occursequentially for each of the images, or preferably the images arecollected on first intermediate transfer member 40 and then themulticolor image is transferred as a whole to second intermediatetransfer member 47 for final transfer to the final substrate 42.

Another embodiment of the apparatus of the invention is shown in FIG. 3Bwhich is identical to the embodiment shown in FIG. 3A except that secondintermediate transfer member 47 has a smaller diameter and inconsequence has less surface area. In this embodiment, secondintermediate transfer member 47 cannot hold at any one moment in timethe complete latent image which is being transferred from firstintermediate transfer member 41. Thus, when the image is multicolor, allthe multicolor images are first collected on the first intermediatetransfer member and only thereafter is the composite image transferredto the second intermediate transfer member.

In this embodiment, the latent image is transferred from the secondintermediate transfer member to final substrate 42 virtuallysimultaneously as it is being transferred to second intermediatetransfer member 47 from first intermediate transfer member 41. Theinventors have discovered that this configuration results in anenhancement of the quality of the image produced on the final substratewhen compared with a configuration in which the second intermediatetransfer member is full-sized. In the latter case, the final substratetends to adhere to the surface of the second intermediate transfermember as the image is being transferred, thereby causing a certainblurring of the image on the final substrate. When the secondintermediate transfer member has a relatively small diameter, preferablyless than 40 mm and more preferably less than 30 mm, the separation ofthe final substrate from the transfer member is improved, there is lesstendency to adhesion, and the quality of the image on the finalsubstrate is thereby enhanced. In particular when the first intermediatetransfer member has a diameter of 70 mm or more, as required to hold anA4 sized image, or a 100 mm diameter or more, as required to hold an A3sized image, optimal results will be obtained when intermediate transfermember 47 has a diameter of less than about 40 mm or less, preferablyabout 30 mm or less.

A duplex embodiment of the invention, for printing two sides of asubstrate at the same time is shown in FIG. 4. The separate color imageswhich make up the multicolored image to be printed on a first side ofsubstrate 42 are first transferred sequentially to intermediate transfermember 40 and then are transferred, preferred as a group, to secondintermediate transfer member 47. Second image transfer member 47 ispreferably heated to a higher temperature than intermediate transfermember 40. The images to be printed on the other side of the page aresubsequently transferred sequentially to intermediate transfer member40, which is meanwhile kept out of the transfer engagement with secondintermediate transfer member 47.

Final substrate 42 is then passed between intermediate transfer member40 and second intermediate transfer member 47, while pressing the twointermediate transfer members together to effect transfer of the imagesto both sides of the paper by heat and pressure. It is understood thatpreferably second intermediate transfer member 47 heats substrate 42 andthe image to a suitable temperature to assure good transfer of the imageon intermediate transfer member 40 to substrate 42. Alternatively oradditionally, the paper may be heated before transfer as described abovein connection with FIG. 2.

In some preferred embodiments of the invention intermediate transfermember 40 acts to heat the image to a first temperature during firsttransfer from photoreceptor 16 to intermediate transfer member 40, andto heat the image to a second higher temperature before second and finaltransfer from intermediate transfer member 40 to final substrate 42.

Exemplary embodiments include the apparatus shown in FIG. 5. Thisapparatus is generally the same as the apparatus of FIG. 1, except thata cooling station 60 is operatively associated with intermediatetransfer member 40 just before it returns to make contact withphotoreceptor surface 16. Intermediate transfer member 40 is cooled atcooling station 60 to locally reduce the temperature of intermediatetransfer member 40 before and during contact with the image on thephotoreceptor. This local cooling allows the liquid toner image to behotter at the point of final transfer from intermediate transfer member40 to final substrate 42 than it is at first transfer from photoreceptorsurface 16 to intermediate transfer member 40.

Cooling station 60 may comprise, for example, apparatus for providing astream of cool air to the surface of the photoreceptor or a cooledroller in contact with the photoreceptor surface. Either or both coolingsystems cool intermediate transfer member 40 to a temperature, higherthan room temperature, but lower than the final transfer temperature.

In a multicolor system, if a roller cooler is used it is coated with anon-stick coating to avoid transfer of the image from intermediatetransfer member 40 to the roller of cooling station 60.

Another exemplary embodiment of this type is illustrated in FIG. 6,which is essentially the same as FIG. 8 of WO 90/04216 previouslyreferenced. Here an intermediate transfer member 140 is of low heatcapacity, and is heated only after first transfer is completed. As shownin FIG. 7, which is the same as FIG. 9 of the above referencedapplication, the temperature at the first transfer is above roomtemperature in order to improve first transfer, and the temperature atsecond transfer is even higher to assure complete or nearly completesecond transfer. For a multi-color system the temperatures and heatcapacities are selected so that the first transfer takes place at atemperature low enough to avoid back transfer.

In the above embodiments, intermediate transfer members 40 and 47 havebeen described as having heaters placed internal to the core to heateach of them to its required temperature. Other methods of heatingintermediate transfer members known in the art can also be used in thepractice of the invention.

EXAMPLES

Colored liquid developer is prepared in the following manner:

Preparation of Black Liquid Developer

10 parts by weight of Elvax 5720 (E. I. Du Pont) and 5 parts by weightof Isopar L are mixed at low speed in a jacketed double planetary mixerconnected to an oil heating unit for one hour, the heating unit beingset at 130° C.

A mixture of 2.5 parts by weight of Mogul L carbon black (Cabot) and 5parts by weight of Isopar L are then added to the mix in the doubleplanetary mixer and the resultant mixture is further mixed for one hourat high speed. 20 parts by weight of Isopar L preheated to 110° C. areadded to the mixer and mixing is continued at high speed for one hour.The heating unit is then disconnected and mixing is continued until thetemperature of the mixture drops to 40° C.

The resulting mixture is transferred to an S-1 attritor device equippedwith 3/16 inch carbon steel media, diluted with Isopar L to a 16% solidsratio and ground without cooling until the temperature rises to about60° C. Cooling, which reduces the temperature to about 30° C. is thencommenced and grinding is continued for a total of 24 hours. The mixtureis removed from the device and diluted with Isopar L to 1.5% by weightsolids concentration. The particles in the resultant toner concentratehave an average diameter of 2.5 microns.

Charge director as known in the art, is added to give the final liquiddeveloper. In a preferred embodiment of the invention the chargedirector of Example 1 of PCT publication WO 90/14617 the disclosure ofwhich is incorporated herein by reference, is added to give the finalliquid developer.

Preparation of Colored Developer

10 parts by weight of Elvax 5720 (E. I. Du Pont) and 5 parts by weightof Isopar L are mixed at low speed in a jacketed double planetary mixerconnected to an oil heating unit for one hour, the heating unit beingset at 130° C.

Pre-heated Isopar L is then added to reduce the solids concentration topreferably 35% and mixing is continued at high speed for one hour. Theheating unit is then disconnected and mixing is continued until thetemperature of the mixture drops to 40° C.

The mixture is then transferred to an S-1 attritor device equipped with3/16 inch carbon steel media and pigment is added to the material in theattritor. The mixture is diluted with Isopar L to about a 12-16% solidsratio, depending on the viscosity of the material and is ground withoutcooling until the temperature rises to about 60° C. Cooling, whichreduces the temperature to about 30° C., is then commenced and grindingis continued for a total of 24 hours. The mixture is removed from thedevice and diluted with Isopar L to 1.5% by weight solids concentration.The particles in the resultant toner concentrate had an average diameterof 2.5 microns.

Charge director as known in the art, is added to give the final liquiddeveloper. In a preferred embodiment of the invention the chargedirector of Example 1 the above referenced PCT publication WO 90/14617is added to give the final liquid developer.

Appropriate colored pigments known in the art of liquid developermanufacture, for example the list given in U.S. Pat. No. 4,794,651 canbe used. Other suitable pigments are Sisco Fast Yellow D1350 (BASF),Lithol Rubin D4576 (BASF), Lyonol blue FG7351 (TOYO) and Lyonol Yellow7G1310 (TOYO). in amounts and combinations depending on the color andintensity required. Optionally, Aluminum Stearate can be added in smallamounts. For pigments which are discolored by steel, other grindingmedia such as zirconia may be used.

These developers are used to form the individual color liquid tonerimages on photoreceptor surface 16 which comprise a relatively highconcentration of toner particles in carrier liquid.

Photoreceptor surface 16 is preferably formed of selenium. Intermediatetransfer member 40 is preferably formed of a cylindrical aluminum corecoated with a 1 nm thick layer of very soft polyurethane having ahardness of 20-25 Shore A. This layer is covered by an offset printingblanket, preferably a KYNIO AIRTRACK offset blanket, which is muchharder than the polyurethane. A thin conducting layer of conductingacrylic covers this layer and is covered in turn by a 0.1 mm layer ofpolyurethane of shore A Hardness 20. This layer is overcoated by a thinlayer of Syl-Off type 291 or 294 silicone release coating.

Liquid developer prepared in accordance with the method described aboveis used in the equipment of FIG. 1. Preferably the temperature of theintermediate transfer layer should be less than about 50° C. Fortemperatures greater than about 50 degrees, there is a tendency for thepreviously transferred colors to back transfer to photoreceptor surface16. Heating intermediate transfer member 40 improves image transfer tointermediate transfer member 40. Intermediate transfer member 40 ispreferably heated to a temperature somewhat below that at which backtransfer begins to occur.

It is believed that the improvement in first transfer when theintermediate transfer member is heated may be a consequence of partialsolvation of carrier liquid by the pigmented toner particles in theimage.

One characteristic of the liquid developers preferred in the practice ofthis invention is that the pigmented toner particles contained thereinsolvate the carrier liquid at elevated temperatures. It is believed thatthere is a partial solvation of the carrier liquid in the tonerparticles during first transfer to heated intermediate transfer member40 which may cause the particles to partially coalesce and form a filmduring first transfer. Coalesced toner is believed to transfer betterthan uncoalesced toner particles.

Furthermore, when the toner material solvates some of the carrierliquid, the toner particles separate from the unsolvated carrier liquid.It is believed that this separated carrier liquid forms a film betweenthe toner image and the photoreceptor which reduces the adhesion of theimage to the photoreceptor, aiding complete transfer of the image to theintermediate transfer member.

It is to be understood that the heating of the image before and/orduring final transfer insures the complete or nearly complete transferof the image from the intermediate transfer member to the finalsubstrate. Where this image heating comes solely by conduction from thepaper, it has been found experimentally that the paper should be at atemperature of at least about 70° C. Higher tempertures such as 80° or90° can also be used, but substantially lower temperatures do nottackify the image enough to assure complete transfer from intermediatetransfer member 40 to paper 42.

The precise temperatures used for particular configurations andcombinations are a function of the material properties of the tonerparticles and the carrier liquid as well as of the quality of therelease layer on the intermediate transfer member. Back transfer occursdue to the tackiness of the image, but is also influenced by therelative adhesion of the image to the release layer on the intermediatetransfer member and to the photoreceptor. It would be possible toincrease the temperature of the intermediate transfer member if therelease properties of the surface of the intermediate transfer memberwere poorer. This however would also result in poorer transfer to thefinal substrate.

In particular representative, operating examples the followingtemperatures are used. In a first example, which is used for thetransfer of single color images, the intermediate transfer member isheated to a surface temperature of 100° C. and the paper is not heated.Calculations show that the image is at a temperature of 52° C. to 63° C.during first transfer. During the interval between first and secondtransfer the image temperature rises to the intermediate transfermember's temperature of 100° C., and the image is cooled during second,final transfer to paper to a temperature of 73° C. to 78° C.

In a second, representative, operating example for sequential transferof multiple images to the intermediate transfer member, the intermediatetransfer member is heated to 50° C. and backing roller 43 is heated to120° C. The image temperature on first transfer is approximately 43° C.and on second transfer it is 75° C. to 78° C.

The temperatures shown in FIG. 7 are also representative of valuessuitable for single image transfer. For multi-image transfer tointermediate transfer member 140, the first transfer temperature must below enough to assure that no back transfer takes place.

It will be understood that certain features and sub-combinations of theinvention are useful, and may be employed without other features andsub-combinations. It is noted that various changes may be made indetails within the scope of the claims without departing from the spiritof the invention. It is therefore to be understood that the invention isnot to be limited to the specific details shown and described.

I claim:
 1. Imaging apparatus for printing an image on a substrate froma latent image formed on a latent image bearing surfacecomprising:developing means (22) for developing the latent image withtoner to form a developed toner image of a given size; a firstcylindrical intermediate transfer member (40) having a first diameter;first transfer means for transferring the developed toner image from thelatent image bearing surface to the first cylindrical intermediatetransfer member; a second cylindrical intermediate transfer member (47);and second transfer means for transferring of the developed image fromthe first intermediate transfer member to the second intermediatetransfer member and from the second intermediate transfer member to thesubstrate, characterized in that the second cylindrical intermediatetransfer member has a diameter not insubstantially smaller than thediameter of the first cylindrical intermediate transfer member. 2.Imaging apparatus according to claim 1 wherein the first cylindricalintermediate transfer member has a surface area large enough toaccommodate the developed toner image.
 3. Imaging apparatus according toclaim 1 wherein the second intermediate transfer member is not largeenough to accomodate the developed image.
 4. Imaging apparatus accordingclaim 1 and including:intermediate transfer member heating means (46,48) for heating the first intermediate transfer member to a firsttemperature and for heating the second intermediate transfer member to asecond temperature higher than the first temperature.
 5. Imagingapparatus according claim 1 wherein the second transfer means includes asecond heating means (43) for heating the substrate.
 6. Imagingapparatus according to claim 5 wherein the second heating meanscomprises a heating backing roller (43) operative to apply heat andpressure to the image during the image transfer.
 7. Imaging apparatusaccording to claim 1 claims and also including first voltage means formaintaining the first intermediate transfer member at a first voltage.8. Imaging apparatus according to claim 7 wherein at least a portion ofthe latent image bearing surface is at a second voltage and the firstvoltage is different from the second voltage.
 9. Imaging apparatusaccording to claim 7 and also including second voltage means formaintaining the second intermediate transfer member at a third voltage.10. Imaging apparatus according to claim 1 wherein the toner is a liquidtoner comprising carrier liquid and toner particles.
 11. Imagingapparatus according to claim 1 wherein the second cylindricalintermediate transfer member has a diameter of less than about 40 mm.12. Imaging apparatus according to claim 1 wherein the secondcylindrical intermediate transfer member has a diameter of less thanabout 30 mm.
 13. Imaging apparatus according to claim 1 wherein transferof the developed image from the second cylindrical intermediate transfermember to the substrate commences before transfer of the developed imagefrom the first intermediate transfer member to the second transfermember is complete.
 14. Imaging apparatus according to claim 1 whereinthe apparatus includes means for producing a plurality of developedimages on the image bearing surface and for transferring the pluralityof developed images to the first cylindrical transfer member in mutualalignment thereon.